Optical fiber amplifiers for amplifying optical signals typically consist of a length of active fiber, which may e.g. be an erbium-doped fiber, and a unit for generating pump power, e.g. a pump laser. When the active fiber is pumped with a strong optical signal (the pump signal) having a wavelength range different from that of the signal to be amplified, and a communications signal is launched into the amplifier, a signal coherent with the signal on the input will occur on the output of the active fiber. The gain is determined i.a. by the power of the pump signal.
The active fiber may be arranged at a considerable distance (e.g. 10-50 km) from the pump laser, in which case the amplifiers are referred to as remote-pumped amplifiers. With e.g. remote-pumped preamplifiers, also called RILP (Remote In-Line Preamplifier), the active fiber is thus spaced from the actual receiver of the optical signals, and it is pumped from the receiver. This takes place via an optical fiber, typically, but not necessarily, the same fiber as transmits the communications signals from the active fiber to the receiver.
The light transmitted in such fibers, in the form of communications signals or pump power, is typically harmful to the human eye. Therefore, because of situations with access to fiber ends or non-connected connectors, it is prescribed by various standards how much optical power may be transmitted from an open fiber end in these situations. These situations may e.g. occur in case of repair, maintenance and testing of systems, or when a fiber has broken, or a connector is disassembled. It is the temporal mean power of the light that is harmful to the eye.
To achieve the desired function of a remote-pumped amplifier, it is necessary to emit levels of pump power in the fiber from the pump laser which significantly exceed the mentioned safety limits. To comply with the safety standards, it is therefore necessary to reduce the pump power in the event that the fiber transmitting the pump power is interrupted between the pump laser and the active fiber.
Further various communications equipment standards prescribe that the equipment must be capable of automatically resuming normal operation when the transmission path has been re-established after a break and transmission signals are transmitted again. For remote-pumped amplifiers, such as e.g. RILP, this requirement, however, is not easy to satisfy, as the reduced pump power results in a considerable reduction in the gain of the active fiber. Therefore, the communications signals arriving at the receiver after the re-establishment of the transmission path, will frequently be below the sensitivity limit of the receiver because of the reduced pump power.
This problem has previously been solved e.g. by using an additional fiber from the receiver to the active fiber. This fiber, in combination with the transmission fiber, is used for passing a control signal from the receiver to the active fiber and back to the receiver. When the control signal is present, there is no break on the fiber and consequently no access to the strong optical pump power, and the pump laser can therefore pump with full power. When, on the other hand, the control signal is absent, this indicates a break on the fiber, involving the risk that the optical power hits an eye, and the pump power is therefore reduced to a safe level until the control signal is present again.
Although this solution is technically adequate, it is vitiated by the serious drawback that it requires an additional fiber typically of a length of 10-50 km. Moreover, a detector or a coupler capable of returning the control signal to the receiver must necessarily be provided at the active fiber.
Systems which are able to reduce the optical output power from a fiber amplifier in case of a broken fiber are also known. These systems do not involve remote-pumped amplifiers and, therefore, they only reduce the power of the communications signals because the pump power never leaves the fiber amplifier itself.
Such a system is described in DE 42 22 270 in which the pump power to the active fiber is reduced if an alarm signal is received from the receiver in the other end of a transmission fiber, said alarm signal indicating that the communications signals are not received, e.g. because of a broken fiber. However, this can only be done if there is an extra fiber or another transmission channel for transfer of the alarm signal and, therefore, this system also has the above-mentioned drawback. Further, the system is not suitable for reducing pump power, unless a special detector unit as above is provided at the active fiber for generation of an alarm signal.
A similar system is known from U.S. Pat. No. 5,428,471 in which two parallel fibers are used for transmission in respective directions. When a fiber amplifier in one direction detects an absent input signal a message is sent via the opposite fiber back to the previous fiber amplifier to reduce or shut down its optical power level. Therefore, also this system has the above-mentioned drawbacks.